If you think that he may be relapsing or has already relapsed, you owe it to him as your brother to try to help him...Even if, like you said, your relationship isn't perfect, you really should at least bring it up with him and ask him if he needs help. He could get upset with you, yes, but deep down he will know that you're worried about his well-being and if anything it could help to bring you two closer.
Why do you believe that he may be back on drugs? What types of behavior is he displaying to make you suspect this?